The present invention relates to a light source for producing an atomic spectral line and particularly to an improved light source provided therein with a hollow cathode.
For the atomic absorption analysis, required is a light source for producing an atomic spectral line having a very narrow line width. A usually used light source produces a spectral line of half width of several GHz (about 2-20 m.ANG.). There has occurred a problem in the conventional light source that a spectral line having a small half width can not be obtained in the case of metal such as Cd, Zn, which is relatively high in vapor pressure and large in oscillator strength (f value) of a resonance line. That is, there may occur a so-called self-absorption phenomenon that the metal vapor excessively generated in the light source absorbs part of emitted light so that the emission line shape lacks its central peak portion, thereby making the half width extremely large. The temperature of the conventional hollow cathode may arise to about 250.degree.-350.degree. C. Low melting point materials may vaporize at such a high temperature and cause self absorption. In the basic research, it has been proposed to cool the light source to suppress the self-absorption phenomenon to thereby obtain a shape emission line of a narrow half width. For example, in Analytica Chimica Acta, 111 (1979), pp. 103-109, K. Tsujii et al. suggest an example in which a double-walled glass tube, through which cooling water is caused to flow, is provided around a hollow electrode in a lamp. Most of metals, except Hg, have only negligibly small vapor pressure below 100.degree. C.
In a practical device, however, it is difficult to realize cooling a lamp by water. That is, not only troublesome piping is required for cooling but also in some cases it is necessary to apply a high potential such as several hundred volts to a cathode which has to be cooled strongly, resulting in incapability of assurance of safety. Further, since it is necessary to replace the light source for each element which is to be analyzed in atomic spectral analysis, there arises a problem that it is required to perform piping operation for the cooling pipes every time the element to be analyzed is changed, in the case where water cooling is performed. It is difficult to replace the light source without allowing a drop of cooling water to escape, thereby causing a problem in the optical system having poor reliability against humidity as well as causing a risk of electric shock because of a high voltage of several hundred volts usually applied in the vicinity of the light source.